You got the whole density story backwards. Lead is lead, if the pellets are harder, it's because the lead has been alloyed, just like for firearm bullets. Alloyed lead has a lower density. Also, using a hexagonal tool to make round holes is asinine. Grind the back of a drill bit or round rod to a nice shape and use that if you must.
You might try a 7/32 bit. That comes to .21875, Which is closer to the .219 that a correctly sized skirt should be. I too have been resizing the JSB's that I find out of round. The 15.89 JSB Match are extremely accurate if everything else is working right.
I just received my first tin of 500 .22 JSB exact jumbo rs diabolo pellets fifteen minutes ago. Tin was tightly packed with no room in to. For pellets to shift around in and the foam disc helped to keep them from shifting during shipping. There were so many dented, malformed and misshaped pellets in the tin had I not ordered them online I would be on my way back inside the store to replace this tin for one that didn’t have even as many subpar pellets in it. I would be less disappointed in the product only if I had paid less for them. So far JSB has not impressed me with anything yet. Just their name used by some shooters who claim to use these pellets. Maybe they are accurate after one goes through and sorts them out.
Not a cheapskate pellet sizers weren't available back then in the U.S. I have them now but this technique worked flawlessly for me back then. Just sharing what worked for me hoping to help others
